Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that can be used to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught, and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to commercial printing and industrial labeling. In either drop-on-demand or continuous inkjet systems, it is necessary to assign a different fluid ink color to a separate printhead. Therefore, in producing color prints, several layers of wet ink may be deposited on a single printed medium.
Continuous inkjet (CU) printers typically comprise two main components, a fluid system and one or more printheads. Ink is delivered through a supply line from a supply reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the printhead. Stimulations are applied to the printhead to cause those ink streams to form streams of spaced droplets, which are deflected into printing or non-printing paths. The non-printing droplets are returned to the supply reservoir via a droplet catcher and a return line. U.S. Pat. Nos. 4,734,711 and 5,394,177 and EP 1,013,450 describe in detail the design of a fluid system for CIJ apparatus. More recent developments of CIJ printing apparatus and printhead fabrication can be found in U.S. Pat. Nos. 6,588,888 and 6,943,037.
Traditional printing presses are able to use high viscosity inks to obtain vibrant, high-density colors. However, high speed inkjet printing and in particular continuous ink jet systems employ low viscosity solutions of dyes or pigments in a water solvent. It is known that increasing the amount of dye or pigment applied to the paper can brighten the colors. However, this process also increases the amount water solvent applied to, and absorbed by, the paper or other media being printed. Absorption of water may cause a paper wrinkling effect called cockle, a wicking and spread of colors referred to as color-to-color bleed, and/or a show-through to the back side of the paper. Efficient drying of inkjet inks on commercial, high speed inkjet printing presses is critical to good image quality. Drying of aqueous inks by unaided evaporation and absorption into the media may not be adequate for high speed presses, particularly when the print media is non-porous and thus poorly absorbing. Auxiliary driers have been proposed to help alleviate this problem.
To remove water from the printed medium, high speed inkjet printing systems such as continuous inkjet systems may utilize an end-of-line dryer that is similar to those used in printing presses. U.S. Pat. No. 5,423,260, e.g., discloses use of an end-of-line dryer to remove water from the printed medium only when all wet ink has been deposited and is at its maximum. It has further been suggested to use infrared lamps or microwave radiation to preferentially heat the ink relative to the unprinted receiver media. Further reductions in the time required between printing and drying have been realized by placing dryers between two printheads to dry the ink before significant amounts of the ink can wick into or otherwise be absorbed by the receiver media. Placement of dryers between printheads is referred to herein as “inter-station drying,” and has been disclosed in, e.g., U.S. Pat. No. 6,428,160. Inter-station drying is effective to provide better optical density, sharper edges, less show through and reduced cockle. In multi-color systems, high-speed dryers placed between the different color printheads reduce color-to-color bleed, and enable more ink to be employed without overly wetting the receiver media.
The use of infrared (IR), and in particular near-infrared (near-IR, or n-IR) (750-1500 nm) drying systems, have been proposed to reduce operating cost and improve productivity by utilizing radiation in the near-infrared part of the spectrum, where electromagnetic radiation for such near-IR systems typically reaches maximum power at a wavelength (lambda max) of approx. 810 nm. When the n-IR radiation is absorbed by the ink and/or substrate, it is converted to heat, thereby increasing the rate of evaporation of the water and optional co-solvents of the inks.
U.S. Pat. No. 5,261,166 discloses a dryer comprising a plurality of infrared burner units with air floatation dryer elements between the infrared units. U.S. Pat. No. 6,412,190 also employs infrared burners in conjunction with air bars. U.S. Pat. No. 6,088,930 employs alternating infrared sources and blower units. WO 88/07103 describes a dryer unit in which the lamp used for generation of infrared radiations enclosed in a box with a reflector behind the lamp and an infrared transmitting window in front of the lamp. U.S. Pat. No. 5,092,059 describes a dryer unit in which an infrared source directs infrared at the paper through a Quartz window. U.S. Pat. No. 6,058,621 describes a dryer in which a plurality of radiant heating bars direct radiation at photosensitive paper. Reflectors are placed behind the infrared lamps. Air flows out between the reflectors, impinging on the paper. US 2009/0031579 discloses a dryer operable with an inkjet printhead which comprises a radiant heat source and an air bearing structure comprising a microporous filter adapted to convert air flow to a diffuse flow.
The use of colloidal silica in inkjet inks has been proposed for various reasons. Typically, such silica particles have been proposed to be employed to affect the physical properties of the resulting printed image. U.S. Pat. No. 5,925,178, e.g., discloses use of colloidal silica to increase optical density and improve abrasion resistance of solid area patches when printed on coated papers and films. U.S. Pat. No. 6,193,792 discloses use of colloidal silica along with a specified surfactant in pigmented inkjet inks. U.S. Pat. No. 6,277,183 discloses the use of silica and metal oxides in inks to decrease drying times, increase water and smear resistance, increase optical density, and provide better quality images with less intercolor bleed. U.S. Pat. No. 7,803,221 discloses use of silica nanoparticles in inks for printing on ceramic surfaces, wherein the inks are fused to the ceramic surfaces upon firing.